What is Second Brain?
A second brain is a trusted digital system, typically a notes app fed by highlights and captures, that stores your ideas, references, and insights outside your head so they can be searched, connected, and reused. The term was popularized by Tiago Forte's methodology and 2022 book Building a Second Brain.
Forte's argument is a division of labor: biological memory is for having ideas, not for holding them. His methodology organizes captured material by actionability using the PARA structure of projects, areas, resources, and archives, and moves notes through a workflow he summarizes as CODE: capture, organize, distill, express. Distillation is done with progressive summarization, layering highlights so a note's core surfaces at a glance.
The concept is best understood as modern packaging for an old lineage that runs from commonplace books through Luhmann's Zettelkasten. What changed is the capture cost: web highlighters, read-later apps, and sync mean the inflow is nearly free, which shifts the real work to distilling and expressing, the steps where generation and retrieval actually produce learning.
A second brain complements rather than replaces the first one. Externalizing reference material frees working memory for thinking, but understanding still has to be built internally through recall, explanation, and use. The systems that work treat the archive as a workshop for future output, not a museum of everything once read.